186 THE POPULAR SCIENCE MONTHLY. 



polar latitudes, and the return, again, of the waters after 10,500 

 years. In this way, and in no other that I can conceive of, can 

 be fairly explained the constant mixture and alternations of terres- 

 trial and marine relics, all through the fossil-bearing formations, 

 and the hundreds, if not thousands of different and distinct strata 

 which are found lying one above another. 



Whoever, even cursorily, studies the phenomena of geology, must 

 be impressed with the enormous length of time it has taken to arrange 

 the terrestrial substructure, and prepare it for the higher forms of 

 life. Even the comparatively recent period of the Bowlder Clay, 

 which laid out the grounds of the present area of civilization, dates 

 back for its commencement, as w r e have seen, probably 200,000 

 years. If it might be assumed that the Permian or New Red 

 Sandstone was formed during the next previous period of extraor- 

 dinary eccentricity, which was 850,000 years ago, then the Devo- 

 nian or Old Red Sandstone would come in, very appropriately, at 

 the next anterior era of extraordinary focal distance, which occurred 

 2,500,000 years back. The Carbonifei-ous period, which came be- 

 tween these two, could not have been formed in less than 1,000,000 

 years, as most geologists concede ; and by calculations previously 

 indicated, those sixty "Welsh laj T ers of coal, if there are that many, 

 divided off by marine deposits of considerable thickness, would have 

 consumed 1,250,000 years. 



The average thickness of all the strata that lie above the Old Red 

 Sandstone is not far from two miles. But this formation is itself, in 

 many places, two miles thick. And the lower Primary systems will 

 add at least ten miles to the vertical measure of the fossil-bearing 

 rocks. It is estimated that "the fossiliferous beds in Great Britain, as 

 a whole, are more than 70,000 feet in thickness ; " and many that are 

 there wanting, or nearly so, elsewhere expand into beds of immense 

 depth. There are certainly fifteen miles deep of strata to be account- 

 ed for the slow accretions of the ages mainly ocean-sediment that 

 has come down from the wear and washings of the solid rocks. It 

 would be by no means a bold assumption to say that 20,000,000 years 

 had elapsed since the eozoon first built its reefs in the warm Lauren- 

 tian seas. 



-+*+- 



AXES AND HATCHETS, ANCIENT AND MODERN. 1 



By the Eev. AET1IUR EIGG, M. A. 



rjTTOOLS with cutting-edges are not only numerous and varied in 

 -L form, but they are also varied in the purposes for which they 

 are formed, and in the mode of using. Hence no very precise state- 

 ment of what is generally meant by a "cutting-edge" can well be 

 1 From a lecture delivered before the London Society of Arts. 



